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Information Regarding Controlling Stockholder Narrative (Details) (Chairman of the Board of Directors and Chief Executive Officer [Member])
Dec. 31, 2014
Common Class A [Member]  
Related Party Transaction [Line Items]  
Ownership in the company 99.00%nwli_CommonStockPercentageOwnership
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_ChiefExecutiveOfficerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
Common Class B [Member]  
Related Party Transaction [Line Items]  
Ownership in the company 33.70%nwli_CommonStockPercentageOwnership
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_ChiefExecutiveOfficerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember